It is already known that if using an NOx adsorbent in which a metal is carried on a carrier which is made of an oxide of at least one metal element selected from Co, Fe, Cu, Ce, and Mn, where the metal carried on the carrier is comprised of a metal which is selected from Cu, Co, Ag, and Pd and which is different from the metal contained in the carrier, and if running through this NOx adsorbent a gas which contains NO and CO, even at an ordinary temperature, the NOx adsorbent will adsorb the NOx (see Patent Literature 1).